558 



VERTEBRAL COLUMN OF MAMMALIA. 



have also appeared two constrictions of the notochord giving 

 rise to a central and to two terminal enlargements. 



On the twelfth day the ossification of the cartilaginous centra 

 commences. 



The first vertebra to ossify is the second or third cervical, 

 and the ossification gradually extends to those behind. It does 

 not commence in the arches till somewhat later than in the 

 bodies. For each arch there are two centres of ossification, one 

 on each side. 



The notochord persists for the greater part of foetal life and 

 even into post-fcetal life. The larger vertebral portions are 

 often the first completely to vanish. They would seem in many 

 cases at any rate (Gegenbaur) to be converted into cartilage, 

 and so form an integral part of the permanent vertebrae. 

 Rudiments of the intervertebral 

 portions of the notochord may 

 long be detected in the ligamenta 

 suspensoria. 



Schwarck (No. 420) states that in both 

 the intervertebral and the vertebral regions, 

 though less conspicuously in the former, 

 the cartilage is divided into two layers, an 

 inner and an outer. He holds that the 

 inner layer corresponds to the cartilaginous 

 notochordal sheath of the lower types, 

 and the outer to the arch tissue. Ossi- 

 fication (Gegenbaur) of the centra appears 

 in a special inner layer of cartilage, which 

 is probably the same as the inner layer of 

 the earlier stage, though this point has 

 not been definitely established. 



FIG. 321. LONGITUDINAL SEC- 

 TION THROUGH THE VERTEBRAL 

 COLUMN OF AN EIGHT WEEKS' 

 HUMAN EMBRYO IN THE THORACIC 

 REGION. (From Kolliker.) 



v. cartilaginous vertebral body ; 

 //. intervertebral ligament; ch, noto- 

 chord. 



Mammalia. The early development of the perichordal 

 cartilaginous tube and rudimentary neural arches is almost the 

 same in Mammals as in Birds. The differentiation into vertebral 

 and intervertebral regions is the same in both groups ; but 

 instead of becoming divided as in Reptilia and Birds into two 

 segments attached to two adjoining vertebrae, the intervertebral 

 regions become in Mammals wholly converted into the interverte- 

 bral ligaments (fig. 322 /*'). There are three centres of ossifica- 

 tions for each vertebra, two in the arch and one in the centrum. 



